The present invention concerns an anti-shock transmission device for driving a rotor of an electric generator by an oscillating weight in a portable apparatus, including a central shaft and an intermediate toothed wheel, one of which is capable of being driven in rotation by movements of the oscillating weight and the other of which is coupled to the generator rotor by a kinematic connection, the central shaft and the intermediate wheel having a common axis of rotation, and a spiral-shaped spring having an inner end coupled to the central shaft and an outer end coupled to the intermediate wheel. The invention also concerns a wristwatch including an electric generator driven by an oscillating weight via such a transmission device.
Such a device can be used in particular in an apparatus of small volume worn by a user whose movements will cause the oscillating weight to swing, usually in rotation. It may be, for example, but in a non-limiting manner, a wristwatch whose watch movement and/or other operating elements are powered by an energy accumulator charged by the electric generator. Since the oscillating weight generally rotates at quite a low speed in normal operation, and the generator rotor has to rotate at a high speed to generate sufficient electric voltage, a multiplying transmission between these elements needs to be used. In cases where the generator rotor includes a single permanent magnet rotating about itself, the necessary transmission ratio is of the order of 100 and it is usually obtained by means of a two stage gear transmission.
Such an arrangement may undergo damage in the event of shock caused for example by the apparatus being accidentally dropped and generating a very high acceleration of the oscillating weight. Despite the small mass of the rotor and the second transmission stage, their inertial creates a not negligible resistant torque because of the high transmission ratio. Such a shock may also permanently damage the transmission or the generator. This is why various anti-shock devices have been proposed for a transmission of this nature.
European Patent No. 0 326 312 proposes a friction transmission between the oscillating weight and the electric generator rotor. The friction is calculated so that the wheel of an intermediate wheel and pinion skids on its shaft when the torque to be transmitted exceeds an admissible value. However, with such a friction device the limit torque value may vary considerably, on the one hand because of the difficulty in obtaining a friction of constant value in large scale manufacturing, and on the other hand because of inevitable variations in friction conditions during use.
European Patent No. 0 791 867 proposes a transmission device of the type indicated in the preamble hereinbefore. In an embodiment illustrated by FIGS. 16 to 18 of the document cited, the inner end of a flat spiral-shaped spring is rigidly fixed to the central shaft driven by the oscillating weight, while its outer end is rigidly fixed to the intermediate wheel which is meshed with the pinion of the generator rotor. The spring has sufficient rigidity to undergo practically no deformation in normal operation. When the oscillating weight is driven slightly more than normal, for example when a watch worn on the wrist is moved abruptly, the spring can be sufficiently deformed to avoid damage to the gears. When there is a more significant acceleration of the oscillating weight, for example if the watch falls to the ground, the spiral spring fastenings are likely to resist. However, the stress which the spiral spring itself undergoes is likely to lead to irreversible deformation or breakage of the spring or its attachments.
The object of the present invention is to perfect an anti-shock transmission device of this type, while avoiding using friction damping and creating a simple inexpensive structure which avoids breaking the spring or its attachment when the apparatus undergoes a shock of a certain amplitude, for example (but in a non limiting manner), up to the value of 5,000 g prescribed by the ISO 1413 standard.
The invention thus concerns an anti-shock transmission device of the type indicated in the preamble, characterised in that the central shaft includes a series of hollows distributed over its periphery and in that the inner end of the spring is arranged to engage in anyone of said hollows via the resilience of the spring and to be able to be released from the hollow when it is subjected to sufficient force in at least one rotational direction of the device.
It may thus be considered that the inner end is coupled to the central shaft by a click mechanism, owing to the spring""s own resilience, this click mechanism being able to be released when the rotation of the shaft in one direction has pushed back the turns of the spring sufficiently outwards for the spring to be no longer able to keep its inner end in the hollow where it was meshed. This end forming a kind of click will then jump out of the hollow to then engage again in the next hollow, as soon as conditions allow. In order to facilitate the release of the end of the spring, each hollow may preferably include a slanting side and the inner end of the spring may include a part which projects inwards, and is able to abut against said slanting side to receive said force and slide over it until said force exceeds a limit value.